This invention relates to wet process plating cells, such as galvanic (for electroplating) or electroless (chemical plating), and is more particularly directed to apparatus and techniques that assist in the plating or wet process treatment of the workpiece to be plated into and from the cell. The invention also concerns a holder or platen that heats the workpiece during a plating operation or other wet-process treatment, which improves the quality of the treated product.
Electroplating plays a significant role in the production of many rather sophisticated technology products, and has recently begun to be used for metallization of semiconductor devices. Recently there has been interest in using plating techniques to form copper conductors on silicon to increase the power or speed of the semiconductor devices. Usually, after copper plating a separate annealing step is used to change the grain structure of the plated material. The separate step requires additional time and additional capital equipment.
A number of techniques for electro-depositing or coating on an article face been described in the patent literature.
A recent technique that employs a laminar flow sparger or injection nozzle within the plating bath is described in my recent U.S. Pat. No. 5,597,460, granted Jan. 28, 1997. The means described there achieve an even, laminar flow across the face of the substrate during the plating operation. A backwash technique carries the sludge and particulate impurities away from the article to be plated, and produces a flat plated article of high tolerance, such as a high-density compact disc master or semiconductor wafer. The techniques in that patent improve the flow regime for the plating solution within the tank or cell, as the flow regime is regarded as being crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it is contacts and flows across the substrate in the plating cell.
In the plating cell as described in said U.S. Pat. No. 5,597,460, a plating bath contains the electrolyte or plating solution, in which the substrate to be plated is submerged in the solution. A sparger or equivalent injection means introduces the solution into the plating bath and forms a laminar flow of the electrolyte or plating solution across the surface of the substrate to be plated. Adjacent the plating bath is an anode chamber in which anode material is disposed, with the material being contained within an anode basket In a typical optical media or semiconductor electrolytic metallization process, the anode material is in the form of pellets, chunks or nuggets of metal, which are consumed during the plating process. A weir separates the plating bath from the anode chamber, and permits the plating solution to spill over its top edge from the plating bath into the anode chamber. The weir is in the form of a semipermeable barrier that permits metal ions to pass through from the anode chamber into the plating bath, but blocks passage of any particulate matter. A circulation system is coupled to the drain outlet to draw off the solution from the anode chamber, together with any entrained particles, and to feed the solution through a microfilter so that all the particles of microscopic size or greater are removed from the plating solution. Then the filtered solution is returned to the sparger and is re-introduced into the plating cell. In this way a backwash of the plating solution is effected, so that the flow regime of the fluid itself washes any particulates out of the anode chamber in the direction away from the plated article. At the same time, the cleansed and purified solution bathes the plated surface of the substrate as a uniform, laminar flow of solution, thus avoiding high spots or voids during plating. As a result, very high tolerance is achieved, permitting production of compact disc or semiconductor device of extreme density without significant error rates.
The flow regime as described in said U.S. Pat. No. 5,597,460 is further improved by the geometry of the well that forms the tank for the plating bath. In that patent the substrate can be positioned on either a fixed or a conventional rotary mount. A conventional cathodic motor rotates the substrate, e.g. at 45-50 RPM. The substrate can be oriented anywhere from vertical to about 45 degrees from vertical. The well has a cylindrical wall that is coaxial with the axis of the substrate. This arrangement was intended to avoid corners and dead spaces in the plating cell, where either the rotation of the substrate or the flowing movement of the plating solution might otherwise create turbulences.
A U-tube laminar flow sparger, shaped to fit on the lower wall of the plating bath or plating cell, can be positioned adjacent the base of the weir to flow the solution into the space defined between the substrate and the weir. The sparger's flow holes are directed in parallel to create a uniform, laminar flow of the electrolyte across the planar face of the substrate. The axes of the flow holes in the sparger define the flow direction of the plating solution, i.e., generally upwards and parallel to the face of the plated substrate.
An increased evenness in plating is achieved by the technique of my earlier copending application, U.S. patent application Ser. No. 09/020,832, filed Feb. 9, 1998, now U.S. Pat. No. 5,932,077 The disclosure in that patent application is incorporated herein by reference. That technique provides an improvement over the technique described and illustrated in my earlier U.S. Pat. No. 5,683,564. According to that improvement, a rotary blade or wiper is positioned in the plating bath between the semipermeable membrane wall and the substrate, and has an edge disposed a predetermined distance from the planar face of the substrate. This distance can be about one-half inch, and is preferably about three-eighths inch. Preferably, the blade or wiper is pitched in the direction such that the rotating wiper tends to pull the electrolyte, plus any hydrogen bubbles, away from the substrate. The rotary wiper can be fluid powered, and as such can be coupled to the electrolyte return conduit so that the electrolyte itself serves as motive power. The fluid powered wiper can be formed with an annular turbine, mounted in a circular mount therefor that is disposed in the plating bath. A circular opening is in registry with the substrate face that is to be plated. The blade on the annular turbine extends radially inwards. The turbine can have vanes around its periphery, and the circular mount can have an annular recess around which the vanes travel. A conduit from the return conduit to the annular recess supplies fluid to propel the turbine and vane. As the same filtered and conditioned electrolyte that is fed through the sparger into the plating bath is also used to power the turbine, the leakage from this turbine does not in any way contaminate or dilute the electrolyte in the plating bath. The same materials that are used in the walls of the plating cell, e.g., a high quality polypropylene or PFA (Teflon), are also used for the rotary blade, turbine, and mount. The annular turbine can be supported for rotation by rollers (formed of the same or a compatible plastic resin) mounted on the support for the annular turbine. This avoids the need for any bearings or metallic parts. In other possible implementations, a different motor mechanism could be employed to rotate the blade or wiper.
Electroless plating is favored in many applications, and especially in those where there is no electrically conductive layer that could serve as a cathode. Accordingly, electroless plating is now seen as an economical alternative to sputtering or vacuum deposition. This is especially true for metals that are difficult to deposit using sputtering or plasma techniques.
One advantageous approach to electroless plating is disclosed in my earlier patent application, Ser. No. 08/873,154, which was filed Jun. 11, 1997 now U.S. Pat. No. 5,865,894. In that arrangement, a megasonic transducer adjacent the floor of the plating cell applies megasonic energy at a frequency of about 0.2 to 5 MHz to the solution. The frequency can be above 1 MHz, and in some cases above 5 MHz. The megasonic waves distribute the solution evenly on the substrate, and also break up any bubbles or concentrations that may lead to defects in the plated surface.
Where the megasonic plating technique is used for electroplating silicon wafers, the flow regime is further improved by rotating the wafers. This can be achieved by placing the wafers in a carrier or boat and rotating the boat, e.g. at 45-50 RPM. This avoids regions of dead flow within the carrier, and results in uniformity of the metallization thickness and quality.
In order to employ the megasonic plating technique with a stationary substrate, the megasonic transducer and the rotary blade can be incorporated together in a plating cell, as described and illustrated in my U.S. patent application Ser. No. 08/954,239, which was filed on Oct. 20, 1997 now U.S. Pat. No. 5,904,827. The disclosure in these patent application Ser. Nos. 08/873,154 and 08/954,239 are incorporated herein by reference.
The technique described in my application Ser. No. 09/020,832, now U.S. Pat. No. 5,932,077 permits mounting the substrate and lowering the substrate into the plating cell to be automated or robotized. Automation and robotization of the insertion, removal, and transport of the workpiece from one process cell to another have been elusive and have not been realized, making it possible to conduct the entire multiple step plating operation in a clean or super-clean environment. In the technique of that application, the carrier for the substrate is disposed on a sealable door for the plating cell. The door opens to a loading position, which is preferably the horizontal position, and closes to a position which preferably holds the substrate vertically in the plating chamber. The door sealably seats onto an opening in a side wall of the cell. An extendible linear actuator, or other equivalent device, can be employed for moving the door between its open and closed positions. The cell favorably incorporates a controllable drain that opens to drain the solution from the cell so that the same is at a level below the door opening when the door is opened, and which closes to permit the cell to be flooded to the lever of the spillover when the door is in its closed position. For electroplating use, a cathode ring may be disposed at the periphery of the door opening for making electrical contact with the substrate when the door is closed. This cathode ring may include a so-called "thieving ring" that extends radially into contact with the substrate.
To date, the substrate temperature has been governed only by the temperature of the electrolyte, or by the heat generated by the plating action itself. No one has provided a platen that permitted the semiconductor wafer or other substrate to be heated above the temperature of the plating bath.
Some heated platens and chucks have been provided for use in vacuum deposition chambers. A few of the many heated chucks and platens for chemical vapor deposition apparatus are described in U.S. Pat. Nos. 5,294,778; 5,648,006; and 5,635,093. There has been no suggestion to incorporate any of their features or principles into a workpiece holder in a wet chemistry treatment.